Reduction of noise



March 28, 1939. w. R. KOCH REDUCTION OF NOISE- Filed May '21, 1936 attorney Hilli- Winfield B.

Patented Mar. 28, 1939 PATENT OFFICE REDUCTION OF NOISE Winfield R. Koch, Merchantville, N. J., assignor to Radio Corporation of America, a. corporation of Delaware Application May 21, 1936, Serial No. 80,996

9 Claims.

This invention relates to the reduction of noise such as that produced in radio receivers and like apparatus by impulses extraneous to the signal.

It has for its principal object the provision of an improved apparatus and method of operation whereby the efiiciency of the signal transmitting channel is varied or controlled to prevent the passage of noise impulses having anamplitude substantially in excess of thatof the signal.

1o Various types of noise reduction systems have been heretofore proposed or used. Some of these systems have involved the use of means for balancing or neutralizing the effect of the impulses which tend-to produce noise. Others have in- I-5 volved the use of a remotely located antenna, a

limiter or peak clipper, special transmission circuit or means for short circuiting; open circuiting or otherwise controlling the transmitting efficiency of the signal channel for the duration of the impulses which tend to produce objectionable noise.

In my copending application, Serial No. 71,348, filed March 28, 1936 is disclosed and claimed a noise reduction system including means which operate for the duration of the noise impulse to interrupt the signal channel and maintain the signal level at the receiver substantially constant during this period. The present invention is similar in some respects to that of the aforesaid application but differs therefrom with respect to the means through which the signal level at the receiver is controlled during the periods of the noise impulses.

The invention will be better understood from the following description when considered in connection with the accompanying drawing and its scope will appear from the appended claims.

Referring to the drawing: 7

Fig. 1 is a wiring diagram of a noise reduction '40 system wherein signal impulses are applied to an amplifier through a capacitor and wherein the discharge of this capacitor through a resistance device such as anelectron discharge tube is varied in accordance with the relatively high amplitude 45 noise impulses;

Fig. 2 is a wiring diagram which illustrates how the system ofFig. 1 maybe modified to produce automatic control of the level at which operation of the resistance device or noise suppressor starts;

and

Fig. 3 is a similar diagram of a somewhat diiferent connection adapted automatically to control the l vel at which the noise suppressor begins to operate.

55 The system of Fig. 1 includes a radio or intermediate frequency input transformer ill from which signal impulses are supplied to an audio frequencyamplifier ll through a channel comprising a detector amplifier tube i2 which may be of the SR? RCA Radiotron type, a delay net- I work l3! l, an amplifier l6 which may be of the 605 RCA Radiotron type and an energy storage device such as a capacitor l5. Output current is supplied from the amplifier ll through the coupling I! to any suitable utilization circuit such 10 as subsequent audio frequency stages and a sound reproducer. It will be understood that power is supplied to the various tube circuits through a negative or ground terminal and a positive termiv, nal indicated by a plus sign. II Connected in shunt to the capacitor l 5 is a tube l8 which may be of the 6J7 RCA Radiotron type and which is controlled in response to relatively high amplitude noise impulses applied through a control channel comprising the diode anode l9 of 20 the tube l2, a resistor 28, biasing element 2l22, capacitor 23 and resistor 24. Timing of the control channel is eiiected by means of a capacitor 25 connected between the anode l9 and ground. A resistor 26 connected to the output terminals of 25 the delay network 53-h! is interposed between ground and the control grid of the tube I8. The arrangement of the system of Fig. 1 will be otherwise understood without detailed explanation. 80

The operation of the system of Fig. 1 will be readily understood. An intermediate frequency impulse applied to the diode anode 2'! of the tube 52 drives its grid 28 more negative thus decreasing its output current, correspondingly decreas- 35 ing the potential drop of the resistor 25 and lowering the potential of the cathode 29. After a time delay dependent on the constants of the network l3-l l, the impulse is applied to the grid of the tube IB thus tending to make the cathode of this tube more negative and to permit discharge of the capacitor I 5. In the case of relatively high amplitude noise impulses, however, this action is prevented by the control chan- -nel which begins to operate as soon as the cathode 29 of the tube l2 becomes suificiently negative to permit the fiow of current between it and the diode anode l9.

When this occurs, current flows through the resistors 20 and 2! to ground, a negative poten- 5 tial is applied to the control grid 39 of the tube l8 through the capacitor 23, the tube I8 is biased to cut-oii'discharge of the capacitor I5 is prevented, and the voltage on the control grid'of the tube II is maintained constant instead of becoming negative with the impulse voltage. Under these conditions, the audio frequency output level is maintained substantially constant for the duration of the noise impulses.

The capacitor 25 connected between the diode anode l9 and ground was found desirable to give a quick-response slow-recovery characteristic to the action of the control channel for preventing the tube l8 from becoming conductive too quickly after the impulse has passed the detector tube but while it is still coming through the time delay network l3l4. The impulse voltage level at which the suppressor or control channel begins to operate is readily adjustable by means of the sliding contact of the resistor 2|. Because the effective output impedance of the tube I6 is quite low when the load is applied to the cathode circuit, the capacitor I may be considerably larger than in the arrangement disclosed by the aforesaid copending application.

In order to effect automatic control of the level at which operation of the control channel begins, the arrangement of Fig. 1 may be modified as illustrated by Fig. 2. This modification involves the provision of a connection 3! through which audio frequency impulses are superimposed on the control channel impulses applied to the resistor 20. With this connection, the tap 32 on tho detector-diode resistor 33 is so adjusted that the 'audio frequency voltage from cathode to ground is equal to that across the diode resistor. The diode l9 will then operate only when the impulse voltage on the cathode is greater than the audio voltage obtained with 100% modulation.

Fig. 3 illustrates a circuit in which automatic control of the voltage level at which the suppressor begins to operate is effected by balancing a direct current component of the signal derived through a diode 34 against an audio frequency component derived from a point 35 preceding the delay network l3l4. It will be noted that the direct current component is applied to the resistor 24 through a resistor 36 and that the audio frequency component is applied to the resistor 24 through the capacitor 23. With these connections, the direct current component tends to drive the control grid of the tube l8 more positive while the audio frequency component tends to drive this grid negative. Strong impulses corresponding to modulation greater than 100% therefore bias the tube 18 beyond cut-ofi, thus preventing discharge of the capacitor I5 as previously explained in connection with the operation of the system illustrated by Fig. l.

I claim as my invention:

1. The combination of signal impulse supply means, an amplifier having an input circuit, energy storage means connected in the input circuit of said amplifier, a signal channel interconnecting said supply means with said storage means to supply energy to said storage means, an impedance device connected in shunt to said storage means for controlling the discharge of energy therefrom, and means interconnecting said supply means and said impedance device for interrupting the discharge of energy from said storage means without interruption of the transmission in said signal channel in response to impulses of an amplitude exceeding a predetermined value.

2. The combination of signal impulse supply means, an amplifier having an input circuit, a capacitor connected in the input circuit of said amplifier, a signal channel interconnecting said supply means with said capacitor to supply energy to be stored to the said capacitor, a thermionic device connected in shunt to said capacitor for controlling the discharge of energy therefrom, and means interconnecting said supply means and said thermionic device for interrupting the discharge of energy from said capacitor without interruption of the transmission in said signal channel in response to impulses of an amplitude exceeding a predetermined magnitude.

3. The combination of signal impulse supply means, an amplifier having an input circuit, a capacitor connected in the input circuit of said amplifier, a signal channel including a delay network interconnecting said supply means with said capacitor, a thermionic device connected in shunt to said capacitor for controlling the discharge of energy therefrom, and means interconnecting said supply means and said device for interrupting the discharge of said energy in response to impulses of an amplitude exceeding a predetermined value.

4. The combination of signal impulse supply means, an amplifier having an input circuit, a capacitor connected in the input circuit of said amplifier, a signal channel including a delay network interconnecting said supply means with said capacitor, a thermionic device connected in shunt to said capacitor for controlling the discharge of energy therefrom, and means interconnecting said supply means and said device for interrupting the discharge of said energy in response to impulses of an amplitude exceeding a predetermined value, and means for timing the action of said thermionic device.

5. The combination of signal impulse supply means, an amplifier having an input circuit, a capacitor connected in the input circuit of said amplifier, a signal channel including a delay network interconnecting said supply means with said capacitor, a thermionic device connected in shunt to said capacitor for controlling the discharge of energy therefrom, and means interconnecting said supply means and said device for interrupting the discharge of said energy in response to impulses of an amplitude exceeding a predetermined value, and means for imparting a quick response and slow recovery characteristic to the action of said thermionic device.

6. The combination of signal impulse supply means, an amplifier having an input circuit, a capacitor connected in the input circuit of said amplifier, means including a delay network connected between said supply means and said capacitor, a variable impedance device connected in shunt to said capacitor, means interposed between said supply means and said device for rendering said device non-conductive in response to impulses of an amplitude exceeding a predetermined value, and signal responsive means connected between said supply means and said de vice for regulating the impulse amplitude at which said device becomes non-conductive.

7. The combination of signal impulse supply means, an amplifier having an input circuit, a capacitor connected in the input circuit of said amplifier, means including a delay network connected between said supply means and said capacitor, a variable impedance device connected in shunt to said capacitor, means interposed be-' tween said supply means and said device for rendering said device non-conductive in response to impulses of an amplitude exceeding a predetermined value, and means responsive to a direct current signal component connected between said supply means and said device for regulating the impulse amplitude at which said device becomes non-conductive.

8. The combination of signal impulse supply means, an amplifier having an input circuit, a capacitor connected in the input circuit of said amplifier, means including a delay network connected between said supply means and said capacitor, a variable impedance device connected in shunt tosaid capacitor, means interposed between said supply means and said device for rendering said device non-conductive in response to impulses of an amplitude exceeding a predetermined value, and means responsive to an audio frequency signal component connected between said supply means and said device for predetermining the amplitude at which said device becomes non-conductive.

9. The combination of signal impulse supply means, an amplifier having an input circuit, a

capacitor connected in the input circuit of said amplifier for transmitting signal impulses from said supply means to said input circuit, a pentode amplifier device connected in shunt to said capacitor, means for biasing said pentode device to cut-off in response to impulses exceeding a predetermined value, manual means for controlling the response of said pentode amplifier device to said impulses, and means controlled by a signal component superimposed on the signal for controlling the response of said pentode amplifier device to said impulses.

WINFIELD R. KOCH. 

